arXiv subjects

L. N. Pfeiffer

Publications and source records attributed to L. N. Pfeiffer.

At least 127 records · Page 7Linked to original sources

arXiv2018

Finite Temperature Behavior in the Second Landau Level of the Two-dimensional Electron Gas

Reports of weak local minima in the magnetoresistance at $ν=2+3/5$, $2+3/7$, $2+4/9$, $2+5/9$, $2+5/7$, and $2+5/8$ in the second Landau level of the electron gas in GaAs/AlGaAs left open the possibility of fractional quantum Hall states at these filling factors. In a high quality sample we found that the magnetoresistance exhibits peculiar features near these filling factors of interest. These features, however, cannot be associated with fractional quantum Hall states; instead they originate from magnetoresistive fingerprints of the electronic bubble phases. We found only two exceptions: at $ν=2+2/7$ and $2+5/7$ there is evidence for incipient fractional quantum Hall states at intermediate temperatures. As the temperature is lowered, these fractional quantum Hall states collapse due to a phase competition with bubble phases.

cond-mat.str-el↗

arXiv2018

Direct observation of composite fermions and their fully spin-polarized Fermi sea near $ν=5/2$

The enigmatic even-denominator fractional quantum Hall state at Landau level filling factor $ν=5/2$ is arguably the most promising candidate for harboring Majorana quasi-particles with non-Abelian statistics and thus of potential use for topological quantum computing. The theoretical description of the $ν=5/2$ state is generally believed to involve a topological p-wave pairing of fully spin-polarized composite fermions through their condensation into a non-Abelian Moore-Read Pfaffian state. There is, however, no direct and conclusive experimental evidence for the existence of composite fermions near $ν=5/2$ or for an underlying fully spin-polarized Fermi sea. Here, we report the observation of composite fermions very near $ν=5/2$ through geometric resonance measurements, and find that the measured Fermi wavevector provides direct demonstration of a Fermi sea with full spin polarization. This lends crucial credence to the model of $5/2$ fractional quantum Hall effect as a topological p-wave paired state of composite fermions.

cond-mat.mes-hall↗

arXiv2018

Superlinear Increase of Photocurrent due to Stimulated Scattering into a Polariton Condensate

We show that when a monopolar current is passed through an n-i-n structure, superlinear photocurrent response occurs when there is a polariton condensate. This is in sharp contrast to the previously observed behavior for a standard semiconductor laser. Theoretical modeling shows that this is due to a stimulated exciton-exciton scattering process in which one exciton relaxes into the condensate, while another one dissociates into an electron-hole pair.

cond-mat.quant-gas↗

arXiv2018

Thermopower and Nernst measurements in a half-filled lowest Landau level

Motivated by recent proposal by Potter et al. [Phys. Rev. X 6, 031026 (2016)] concerning possible thermoelectric signatures of Dirac composite fermions, we perform a systematic experimental study of thermoelectric transport of an ultrahigh-mobility GaAs/AlxGa1-xAs two dimensional electron system at filling factor v = 1/2. We demonstrate that the thermopower Sxx and Nernst Sxy are symmetric and anti-symmetric with respect to B = 0 T, respectively. The measured properties of thermopower Sxx at v = 1/2 are consistent with previous experimental results. The Nernst signals Sxy of v = 1/2, which have not been reported previously, are non-zero and show a power law relation with temperature in the phonon-drag dominant region. In the electron-diffusion dominant region, the Nernst signals Sxy of v = 1/2 are found to be significantly smaller than the linear temperature dependent values predicted by Potter et al., and decreasing with temperature faster than linear dependence.

cond-mat.mes-hall↗

arXiv2018

Surface segregation and the Al problem in GaAs quantum wells

Low-defect two-dimensional electron systems (2DESs) are essential for studies of fragile many-body interactions that only emerge in nearly-ideal systems. As a result, numerous efforts have been made to improve the quality of modulation-doped Al$_x$Ga$_{1-x}$As/GaAs quantum wells (QWs), with an emphasis on purifying the source material of the QW itself or achieving better vacuum in the deposition chamber. However, this approach overlooks another crucial component that comprises such QWs, the Al$_x$Ga$_{1-x}$As barrier. Here we show that having a clean Al source and hence a clean barrier is instrumental to obtain a high-quality GaAs 2DES in a QW. We observe that the mobility of the 2DES in GaAs QWs declines as the thickness or Al content of the Al$_x$Ga$_{1-x}$As barrier beneath the QW is increased, which we attribute to the surface segregation of Oxygen atoms that originate from the Al source. This conjecture is supported by the improved mobility in the GaAs QWs as the Al cell is cleaned out by baking.

cond-mat.mes-hall↗

arXiv2018

Stability of the 5/2 Fractional Quantum Hall State in a Corbino Disc Sample with In-plane Electric Fields

Among the various geometries to study the fractional quantum Hall effect in two dimensional electron gas, the Corbino disc owns the advantage to probe the bulk properties directly. In this work we explore the influence of in-plane electric fields on the stability of the 5/2 fractional quantum Hall state realized in Corbino geometry. The effect of weak electric fields is investigated at ultra-low temperatures in order to compare with a theoretical proposal of enhanced Pfaffian state under weak electric fields.

cond-mat.mes-hall↗

arXiv2018

Intersubband scattering in n-GaAs/AlGaAs wide quantum wells

Slow magnetooscilations of the conductivity are observed in a 75 nm wide quantum well at heating of the two-dimensional electrons by a high-intensity surface acoustic wave. These magnetooscillations are caused by intersubband elastic scattering between the symmetric and asymmetric subbands formed due to an electrostatic barrier in the center of the quantum well. The tunneling splitting between these subbands as well as the intersubband scattering rate are determined.

cond-mat.mes-hall↗

arXiv2018

Cyclotron Orbits of Composite Fermions in the Fractional Quantum Hall Regime

We study a bilayer GaAs hole system that hosts two distinct many-body phases at low temperatures and high perpendicular magnetic fields. The higher-density (top) layer develops a Fermi sea of composite fermions (CFs) in its half-filled lowest Landau level, while the lower-density (bottom) layer forms a Wigner crystal (WC) as its filling becomes very small. Owing to the inter-layer interaction, the CFs in the top-layer feel the periodic Coulomb potential of the WC in the bottom-layer. We measure the magnetoresistance of the top-layer while changing the bottom-layer density. As the WC layer density increases, the resistance peaks separating the adjacent fractional quantum Hall states in the top-layer change nonmonotonically and attain maximum values when the cyclotron orbit of the CFs encloses one WC lattice point. These features disappear at T = 275 mK when the WC melts. The observation of such geometric resonance features is unprecedented and surprising as it implies that the CFs retain a well-defined cyclotron orbit and Fermi wave vector even deep in the fractional quantum Hall regime, far from half-filling.

cond-mat.mes-hall↗

arXiv2018

Split-gate device for indirect excitons

We present a concept and experimental proof of principle for split-gate devices for indirect excitons (IXs). The split-gate forms a narrow channel, a point contact, for IX current. Control of IX flow through the split-gate with both gate voltage and excitation power is demonstrated.

cond-mat.mes-hall↗

arXiv2017

Competing charge density waves probed by non-linear transport and noise in the second and third Landau levels

Charge density waves (CDW) in the second and third Landau levels (LL) are investigated by both non-linear electronic transport and noise. The use of a Corbino geometry ensures that only bulk properties are probed, with no contribution from edge states. Sliding transport of CDWs is revealed by narrow band noise in re-entrant quantum Hall states R2a and R2c of the second LL as well as in pinned CDWs of the third LL. Competition between various phases - stripe, pinned CDW or fractional quantum Hall liquid - in both LL are clearly revealed by combining noise data with maps of conductivity versus magnetic field and bias voltage.

cond-mat.mes-hall↗

arXiv2017

Berry Phase and Anomalous Transport of the Composite Fermions at the Half-Filled Landau Level

The fractional quantum Hall effect (FQHE) in two-dimensional electron system (2DES) is an exotic, superfluid-like matter with an emergent topological order. From the consideration of Aharonov-Bohm interaction of electrons and magnetic field, the ground state of a half-filled lowest Landau level is mathematically transformed to a Fermi sea of composite objects of electrons bound to two flux quanta, termed composite fermions (CFs). A strong support for the CF theories comes from experimental confirmation of the predicted Fermi surface at $ν$ = 1/2 (where $ν$ is the Landau level filling factor) from the detection of the Fermi wave vector in the semi-classical geometrical resonance experiments. Recent developments in the theory of CFs have led to a prediction of a $π$ Berry phase for the CF circling around the Fermi surface at half-filling. In this paper we provide the first experimental evidence for the detection of the Berry phase of CFs in the fractional quantum Hall effect. Our measurements of the Shubnikov-de Haas oscillations of CFs as a function carrier density at a fixed magnetic field provide a strong support for an existence of a $π$ Berry phase at $ν$ = 1/2. We also discover that the conductivity of composite fermions at $ν$ = 1/2 displays an anomalous linear density dependence, whose origin remains mysterious yet tantalizing.

cond-mat.mes-hall↗

arXiv2017

Simultaneous Measurements of Microwave Photoresistance and Cyclotron Reflection in the Multi-Photon Regime

We simultaneously measure photoresistance with electrical transport and plasmon-cyclotron resonance (PCR) using microwave reflection spectroscopy in high mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field. Multi-photon transitions are revealed as sharp peaks in the resistance and the cyclotron reflection on samples with various carrier densities. Our main finding is that plasmon coupling is relevant in the cyclotron reflection spectrum but has not been observed in the electrical conductivity signal. We discuss possible mechanisms relevant to reflection or dc conductivity signal to explain this discrepancy. We further confirm a trend that higher order multi-photon features can be observed using higher carrier density samples.

cond-mat.mes-hall↗

arXiv2017

Low-Frequency Microwave Induced Quantum Oscillations in A Two-Dimensional Electron System

We study the magnetoresistance of an ultrahigh mobility GaAs/AlGaAs two-dimensional electron sample in a weak magnetic field under low-frequency (f < 20 GHz) microwave (MW) irradiation. We observe that with decreasing MW frequency, microwave induced resistance oscillations (MIRO) damp and multi-photon processes become dominant. At very low MW frequency (f < 4 GHz), MIRO disappears gradually and a new SdH-like oscillation develops. The analysis indicates that the new oscillation may originate from alternating Hall-field induced resistance oscillations (ac-HIRO), or can be viewed as a multi-photon process of MIRO in low MW frequency limit. Our findings bridge the non-equilibrium states of MIRO and HIRO, which can be brought into a frame of quantum tunneling junction model.

cond-mat.mes-hall↗

arXiv2017

Interaction-induced Interlayer Charge Transfer in the Extreme Quantum Limit

An interacting bilayer electron system provides an extended platform to study electron-electron interaction beyond single layers. We report here experiments demonstrating that the layer densities of an asymmetric bilayer electron system oscillate as a function of perpendicular magnetic field that quantizes the energy levels. At intermediate fields, this interlayer charge transfer can be well explained by the alignment of the Landau levels in the two layers. At the highest fields where both layers reach the extreme quantum limit, however, there is an anomalous, enhanced charge transfer to the majority layer. Surprisingly, when the minority layer becomes extremely dilute, this charge transfer slows down as the electrons in the minority layer condense into a Wigner crystal. Furthermore, by examining the quantum capacitance of the dilute layer at high fields, the screening induced by the composite fermions in an adjacent layer is unveiled. The results highlight the influence of strong interaction in interlayer charge transfer in the regime of very high fields and low Landau level filling factors.

cond-mat.mes-hall↗

arXiv2017

Observation of an anomalous density-dependent energy gap of the $ν=5/2$ fractional quantum Hall state in the low density regime

We have studied the $ν=5/2$ fractional quantum Hall state in a density-tunable sample at extremely low electron densities. For the densities accessed in our experiment, the Landau level mixing parameter $κ$ spans the $2.52<κ<2.82$ range. In the vicinity of $5.8 \times 10^{10}$~cm$^{-2}$ or $κ= 2.6$ an anomalously large change in the density dependence of the energy gap is observed. Possible origins of such an anomaly are discussed, including a topological phase transition in the $ν=5/2$ fractional quantum Hall state.

cond-mat.mes-hall↗

arXiv2017

Stability of the $N = 2$ bubble states in Corbino geometry under a tilted field

We measured the activation energy gap of the N = 2 bubble states using tilted-field technique in two GaAs/AlGaAs samples with Corbino geometry, and observed monotonically decreasing energy gaps with the increasing total magnetic field. The observed tilt angle dependence is discussed based on magneto-orbital coupling and finite-size effect. We also found a bubble state at an uncommon filling factor of 4.67, with an unanticipated energy gap of up to 5.35 K and opposite in-plane field dependence.

cond-mat.mes-hall↗

arXiv2017

Specific heat and entropy of fractional quantum Hall states in the second Landau level

Specific heat has had an important role in the study of superfluidity and superconductivity, and could provide important information about the fractional quantum Hall effect as well. However, traditional measurements of the specific heat of a two-dimensional electron gas are difficult due to the large background contribution of the phonon bath, even at very low temperatures. Here, we report measurements of the specific heat per electron in the second Landau level by measuring the thermalization time between the electrons and phonons. We observe activated behaviour of the specific heat of the 5/2 and 7/3 fractional quantum Hall states, and extract the entropy by integrating over temperature. Our results are in excellent agreement with previous measurements of the entropy via longitudinal thermopower. Extending the technique to lower temperatures could lead to detection of the non-Abelian entropy predicted for bulk quasiparticles at 5/2 filling

cond-mat.mes-hall↗

arXiv2017

Spin-orbit coupling and transport of strongly correlated two-dimensional systems

Measuring the magnetoresistance (MR) of ultraclean {\it GaAs} two-dimensional holes in a large $r_s$ range of 20-50, two striking behaviors in relation to the spin-orbit coupling (SOC) emerge in response to strong electron-electron interaction. First, in exact correspondence to the zero-field metal-to-insulator transition (MIT), the sign of the MR switches from being positive in the metallic regime to being negative in the insulating regime when the carrier density crosses the critical density $p_c$ of MIT ($r_s\sim 39$). Second, as the SOC-driven correction $Δρ$ to the MR decreases with reducing carrier density (or the in-plane wave vector), it exhibits an upturn in the close proximity just above $p_c$ where $r_s$ is beyond 30, indicating a substantially enhanced SOC effect. This peculiar behavior echoes with a trend of delocalization long suspected for the SOC-interaction interplay. Meanwhile, for $p 40$, in contrast to the common belief that a magnet field enhances Wigner crystallization, the negative MR is likely linked to enhanced interaction.

cond-mat.str-el↗